1. Field of the Invention
The invention relates generally to methods for determining the focal properties of a patient's eye, and relates more particularly to methods for determining the spherical and astigmatic refractive correction for a patient's eye.
2. Prior Art
U.S. Pat. Nos. 3,822,932 and 3,947,097 issued to W. E. Humphrey describe novel apparatus for determining the refractive correction for a patient's eye. In addition, U.S. Pat. No. 3,947,097 describes two basic embodiments of a method for using the described apparatus. Briefly, the apparatus and method allow the spherical and astigmatic portions of the refractive correction to be determined somewhat independently of one another. Furthermore, and most importantly, the astigmatic portion of the refractive correction is specifically determined as the combination of two astigmatic components which are measured independently of one another. Simple line targets with fixed orientations are used in the measurement process, greatly simplifying the patient's task. The patient is only required to judge the best focus of each line target as the correcting optical elements are varied.
One embodiment of Humphrey's method as described in U.S. Pat. No. 3,947,097 uses apparatus having a single variable astigmatic sub-system. This apparatus is used to measure the two astigmatic components successively and completely independently from one another. The complete astigmatic correction must then be calculated mathematically or graphically. This embodiment does not provide for optically generating or simulating the complete refractive correction whereby a corrected visual acuity may be obtained by the patient viewing a visual acuity chart through the simulated refractive correction.
The preferred embodiment of Humphrey's method as described in U.S. Pat. No. 3,947,097 uses apparatus which includes two separate variable astigmatic optical sub-systems which are used respectively to measure the two astigmatic components. This apparatus actually optically generates the complete refractive correction during the measurement process, allowing corrected visual acuity to be determined through the simulated refractive correction. This preferred embodiment of Humphrey's method is advantageous not only because visual acuity may be readily obtained, but also because the astigmatic refractive correction may be easily refined by convenient repetition of the steps in the measurement process. However, there are also disadvantages of Humphrey's preferred embodiment. Although the complete astigmatic correction is optically generated, to express this correction in conventional form still requires either mathematical or graphical calculation. In addition, subsequent arbitrary adjustment of the conventional form of the refractive correction cannot be accomplished without mathematical or graphical calculation to convert the arbitrary adjustment into the proper combination of adjustments for the two variable astigmatic sub-systems. Furthermore, apparatus having two separate variable astigmatic sub-systems is not common in the art. In addition to requiring an excessive number of optical elements, such apparatus tends to introduce additional optical aberrations, and for large refractive corrections the mathematical combination of the powers of the optical elements becomes non-linear. Certain aspects of Humphrey's preferred embodiment will later be described in detail.
There are thus several reasons why apparatus having a single variable astigmatic sub-system is preferable to apparatus having two separate such sub-systems. Examples of apparatus having a single variable astigmatic sub-system are numerous in the art. See, for example, U.S. Pat. No. 579,132 issued to F. M. Clark; U.S. Pat. No. 735,460 issued to W. L. Burnam; U.S. Pat. No. 1,070,631 issued to G. A. Rogers; Asher, H.M.F., Journal of Physiology 134:4P-5P (1956); and U.S. Pat. No. 3,664,631 issued to D. Guyton. Each apparatus of this type, in order to optically generate and simulate the complete refractive correction, includes means for rotating the variable astigmatic sub-system about the optical axis of the apparatus in order that the principal meridians of the astigmatic sub-system may be aligned with the principal meridians of the eye's astigmatic refractive error. Location of the principal meridians of the eye's astigmatic refractive error is difficult. Methods for locating these principal meridians often involve the use of targets difficult for the patient to interpret. Such methods are tedious for both the patient and the examiner.
Humphrey's method for determining the refractive correction, as described in his U.S. Pat. No. 3,947,097, uses exceedingly simple targets for the patient to interpret, that is, straight line targets with fixed orientations. The principal meridians of the eye's astigmatic error are not located initially but are located by calculation at the conclusion of the procedure. However, as stated previously, when Humphrey's method is used with apparatus having only one variable astigmatic sub-system, the complete refractive correction is not optically generated or simulated during the measurement procedure, but rather the complete refractive correction is mathematically calculated at the conclusion of the procedure. To obtain the patient's corrected visual acuity, the apparatus would have to be further adjusted to simulate the complete refractive correction according to the results of the mathematical calculations. Furthermore, when using apparatus having only one variable astigmatic sub-system. Humphrey provides no technique to further refine the refractive correction; the accuracy of the refractive correction may only be judged by repetition of the entire procedure.